http://d.puremagic.com/issues/show_bug.cgi?id=11235

           Summary: Add analog of TypeTuple that does not auto-expand
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: Phobos
        AssignedTo: nobody@puremagic.com
        ReportedBy: public@dicebot.lv


--- Comment #0 from Dicebot <public@dicebot.lv> 2013-10-12 14:23:58 PDT ---
This topic pops up frequently in D.learn when one needs to pass several distinct TypeTuple's as template arguments. Auto-expansion behavior of existing TypeTuple does not make it possible because two will be merged into single argument list.

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Dicebot <public@dicebot.lv> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |pull


--- Comment #1 from Dicebot <public@dicebot.lv> 2013-10-12 14:26:27 PDT ---
https://github.com/D-Programming-Language/phobos/pull/1633

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Andrej Mitrovic <andrej.mitrovich@gmail.com> changed:

           What    |Removed                     |Added
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                 CC|                            |andrej.mitrovich@gmail.com


--- Comment #2 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2013-10-12 14:34:46 PDT ---
Isn't this what http://dlang.org/phobos/std_typecons.html#.Tuple is for?

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--- Comment #3 from Dicebot <public@dicebot.lv> 2013-10-12 14:50:38 PDT ---
No, those are quite different. `std.typecons.Tuple` defines runtime tuple
(struct). It can be CTFE'ed of course, but it can't contain types (in TypeTuple
sense) or aliases (completely), only values and value meta-data.

Proposed `Group` acts exactly like `TypeTuple` but do not auto-expand.

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--- Comment #4 from Dicebot <public@dicebot.lv> 2013-10-12 14:52:15 PDT ---
If TypleTuple comparison would have been legal, one could say that `Group!(int,
double, foo).expand == TypeTuple!(int, double, foo)`

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--- Comment #5 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2013-10-12 14:55:06 PDT ---
(In reply to comment #3)
> No, those are quite different. `std.typecons.Tuple` defines runtime tuple
> (struct). It can be CTFE'ed of course, but it can't contain types (in TypeTuple
> sense) or aliases (completely), only values and value meta-data.
> 
> Proposed `Group` acts exactly like `TypeTuple` but do not auto-expand.

Right.. well with Group you end up having a third tuple kind. I think it's best that we wait for a language implementation of tuple syntax (see http://wiki.dlang.org/DIP32).

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--- Comment #6 from Dicebot <public@dicebot.lv> 2013-10-12 15:05:25 PDT ---
It is not a 3d kind of tuple, it is just another syntax for existing TypeTuple. Considering outcome of last tuple discussion threads any changes in that domain won't come any time soon and this small addition allows to implement certain algorithms right here and now.

I personally don't mind keeping it in "stdx" part of my projects but it is mentioned quite frequently as a necessary feature [citation needed].

Also DIP32 addresses different issues and won't change the fact that both expanding and non-expanding tuples are needed in different places.

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David Nadlinger <code@klickverbot.at> changed:

           What    |Removed                     |Added
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                 CC|                            |code@klickverbot.at


--- Comment #7 from David Nadlinger <code@klickverbot.at> 2013-10-12 15:45:59 PDT ---
I wonder whether it is worth adding this as an officially supported kind of tuple, or if we should encourage making creative use of the current syntax to keep the complexity low: (untested, just typed it up to illustrate the principle of nesting templates)

---
template isEqual(T...) {
  template to(U...) {
    static if (T.length == U.length) {
      static if (T.length == 0) {
        enum to = true;
      } else {
        enum to = isSame!(T[0], U[0]) && isEqual!(T[1 .. $]).to!(U[1 .. $])
      }
    } else {
      enum to = false;
    }
  }
}

static assert(isEqual!(1, 2, 3).to!(1, 2, 3));
---

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--- Comment #8 from Dicebot <public@dicebot.lv> 2013-10-13 03:48:30 PDT ---
Main problem with nesting is that it does not allow algorithms with variadic amount of tuples.

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